My 996 doesn't like slopes - but only when its warm!

Hi, New to the forum, so thanks for taking a look. I have a March 1998 996 (so one of the early ones), 86K, good condition, full history, had it a couple of months and all seems as well as can be expected. However, one niggle which is driving me nuts. My driveway slopes down towards the street, not massively, perhaps 1:25. I park the car nose up the slope (usually). From cold the car starts no problem. Drive it around (okay go for a burn..!) park it up level on the street - no problem, restarts every time. (In fact when its hot it will start on the slope OK for a few minutes....). BUT let it cool down for about an hour, so not fully back to cold, it is the devils own job to start with much swearing and teasing the throttle until it fires - once started its good as gold again. Now this is where it gets really cookie - reverse it onto the drive so its pointing nose down the slope, the problem doesn't occur. So to recap, warm (or hot) engine, parked on the level or nose down the slope, no problem. Warm engine, nose up the slope, poor starting. Diagnostics shows nothing; even cleared the buffer down, replicated the problem, and interrogated again (at the dealer) - still no clue. As a precaution we have changed the ICV, the temp sensor, the impulse sensor, and it had a new air MAF sensor not long before I bought it (all confirmed by diagnostics) - can't think of much else, except perhaps the starter which incidentally can be screechy, but again only when on the slope..... However, the starter otherwise seems strong. Did wonder about the fuel pump/fuel pump relay, but that's venturing into cash I'd rather not spend territory, unless I can be sure it would fix the problem. The inclined angle somehow plays a part so I'm speculating that if its not electronics, it might be some sort of physical/mechanical problem - is there anything like a non return valve in the fuel flow that could be susceptible I wonder? Any suggestions or similar experience (and resolution!) much appreciated. Thanks.

Being difficult to start when on an incline isn't something I've heard of before. More commonly, people will complain about the car smoking when starting it when the rear has been facing down an incline. This has to do with the engine design and the way that it can allow oil to seep into the cylinders, resulting in the puff of smoke when starting the car.

Thanks for that - nope, no puff of smoke - no clue at all unfortunately - I'm a member of Porsche Club GB and their tech gurus are scratching their heads too - dealers are equally non-plussed. There has been murmurings of non return valves in the fuel line, bu no-one seems to be able to say for sure what or where these are - and again why the problem only manifests under warm engine conditions. I did wonder if the ECU may somehow be confusing warm with cold and squirting too much fuel along the line, but then again why only on the incline, not on the level? Only other slight clue is that intermittently the car will fire up and then immediately fail to idle and stall - corrected by a restart and a blip on the throttle. That can happen under hot or cold conditions - that's why I changed the ICV, but that problem still presents - again I suppose it might be indicating some sort of inconsistency in the fuel flow on start. Further best guesses more than welcome!

This definitely calls for a durametric and some data logging. Short of "the right tool for the job" you're going to be spending a lot of time and money guessing in the dark.

Test fuel pressure at the fuel rail. Might want to fill tank full of petrol & try to duplicate the issue 1st if pressure gauge is not readily available.

^^^ As Bypro says ^^^ Has the filter been replaced at all ?? The only other thing that gets touchy with age and heat is the Crank Position Sender - but why it would /would not work on an incline ???

Except for the slope an inoperative purge valve in the engine compartment has been known to cause hard starting after stop for fuel. IE: hot engine has cooled but not cold enough to call for enrichment. Purge valve in pic 3-4 post 103 here on page 7 https://rennlist.com/forums/996-foru...em-pics-7.html

I would start by confirming the slope hypothesis. I know you have some observations of it happening, but it seems unlikely, so I would run a series of controlled tests to confirm there is a correlation. If there isn't, then you can focus on electrical, the in particular the power cable running from the alternator that is known to cause hot start issues. If it is, then you can focus on fuel related issues (starting with Bypro's advice). -S. PS--Just noticed a thread below this one discussing the cable I mentioned. Take a look: https://rennlist.com/forums/996-foru...ter-cable.html

Thanks everybody for these really helpful suggestions - I think we can rule out the crank sensor as that has been replaced. The alternator/starter cable and the fuel line issues will be investigated - and the purge valve is a new suggestion so that's got to be worth a look too. Will let you know how I get on.